Tag Archives: letter-materials-top

Topological insulators, a class of materials which has been investigated for just over a decade, have been heralded as a new ‘wonder material’, as has graphene. But so far, topological insulators have not quite lived up to the expectations fueled by theoretical studies. University of Groningen physicists now have an idea about why. Their analysis was published on 27 July in the journal Physical Review B.

Topological insulators are materials that are insulating in the bulk but allow charge to flow across the surface. These conducting states at the surface originate from ordering patterns in the states where electrons reside that are different from ordinary materials. This ordering is linked to the physical concept of ‘topology’, analogous to that used in mathematics. This property gives rise to very robust states with some special properties.

Heavy atoms

For one, their spin — a magnetic property of electrons which can have the values ‘up’ or ‘down’ — is locked to their movement. ‘This means that electrons moving to the right have spin down, and those moving to the left have spin up’, explains first author of the study Eric de Vries, PhD student in the ‘Spintronics of Functional Materials’ research group led by his supervisor prof. dr. Tamalika Banerjee. This is group is part of the Zernike Institute for Advanced Materials. ‘But it also means that when you inject electrons with spin up into such a topological insulator, they will travel to the left!’ Topological insulators might therefore be very useful in the realization of spintronics: electronics based on the quantized spin value rather than the charge of electrons.

The special properties of topological insulators are predicted by the theoretical analysis of the surface structures of these materials, made from crystals of heavy atoms. But experiments show mixed results, which don’t quite live up to the theoretical predictions. ‘We wondered why, so we devised experiments to investigate the behaviour of the surface state electrons. Specifically, we wanted to see if transport is really limited to the surface, or if it is also present in the bulk of the material.’

Surprising

Earlier experiments by the group, in which they used ferromagnets to detect the spins of electrons generated in the topological insulator, were surprising, says De Vries. ‘We demonstrated that a voltage presumably originating from spin detection can originate in factors other than the locking of electron spin to its movement. Using different geometries, we showed that artefacts related to stray magnetic fields generated by the ferromagnets can mimic similar spin voltages.’ This observation may lead to a re-evaluation of some published results.

This time, they used a different approach. ‘We analyzed the topological insulators using strong magnet fields. This causes electrons to oscillate in transport channels.’ De Vries went to the national High Field Magnet Laboratory at the Radboud University Nijmegen, where a 33-Tesla magnet is available, one of the stronger magnets in the world. ‘Others have done similar tests with weaker magnets, but these are not sensitive enough to reveal the additional transport channels that coexist with the surface states.’ De Vries’s experiments showed that a considerable part of the charge transport occurred in the bulk phase of the material, and not only at the surface.

Transport channels

The reason for this, explains De Vries, is the imperfect crystal structure of the topological insulator. ‘Sometimes there are atoms missing in the crystal structure. This results in freely moving electrons. These start to conduct as new transport channels, generating electric current in the bulk of the material.’

So why has no one noted this before? De Vries stresses that interpreting transport measurements made on topological insulators can be difficult. ‘We experienced this in our previous experiments. Our message is that extreme care is needed in the interpretation of experimental observations for devices based on these materials.’ Also, experiments which might lead to clearer conclusions require very high magnetic fields in specialized labs.

Glitches

The results point to a way to improve topological insulators. ‘The key is to grow the crystals without any missing atoms. Another solution is to fill the holes, for example with calcium ions that bind the free electrons. But that might cause other disturbances to the electrons’ mobility.’ For ten years, topological insulators were all the rage. They were compared to the wonder material graphene. The discovery that, in practice, topological insulators have glitches serves as a reality check. De Vries: ‘We need to study and understand the interaction between the surface states and the bulk material in much more detail.’

The III-N semiconductor family has attracted significant research attention over the last 25 years, resulting in intensive patenting activity, with a substantial increase during the past decade. More than 80,000 patents and patent applications related to III-N technology have been published worldwide since the early 1990s, announce KnowMade’s analysts. In such a dynamic III-N market, it is essential to understand the technology challenges and the market needs as well as to track related patents. Therefore, industrial companies need to anticipate changes, quickly detect business opportunities, mitigate risks, and make strategic decisions.

KnowMade, System Plus Consulting and Yole Développement, all part of Yole Group of Companies combine their expertise to develop relevant services and high-added value reports dedicated to the III-N technology. Based on technology changes, market evolution and IP strategy, the group is covering the overall GaN industry from LED, diode and laser to RF applications as well as other III-N materials. What is the status of the III-N semiconductor field? Yole Group of Companies proposes an overview of this industry.

III-nitride

The Technology Intelligence & IP strategy consulting company, KnowMade presents today a new service to follow the industry evolution and get a comprehensive understanding of the technical challenges and company’s market positioning through an IP approach. III-N Patent Watch service is monthly updates dedicated to the III-N related patents. With a useful Excel database presenting the latest patent applications, newly granted patents, expired or abandoned, patent transfers and patent litigation and more, the Patent Watch service is a powerful tool of strategic analysis to track competitors, partners and customers and identify new entrants. Patent Watch also allows companies to identify business opportunities as well as analyze the risks for business development.

Under this service, the technology intelligence and IP strategy consulting company is tracking the IP of more than 100 players involved in the III-N sector. The take-off of patenting activity took place in the 2000s with a first wave of patent publications. A second wave started in 2010 while first commercial GaN products, collaborations, mergers, and acquisitions emerged… III-N Patent Watch service from KnowMade help the companies to get a clear view of the market evolution, understand the IP strategies, and anticipate the industry changes and much more.

In parallel, System Plus Consulting and Yole Développement are strongly involved in the GaN industry, representing the biggest market of the III-N semiconductor materials family. Both companies propose a huge collection of reverse engineering and costing analyses and technical and market reports to highlight the technology innovations, markets adoption and give a quantification of these markets. According to Yole Développement, the global GaN market including LED, RF, Power and laser, was estimated to be worth US$16 Billion in 2016 and should reach US$20 Billion by 2020 at a 5% CAGR between 2016 and 2020. Indeed the overall GaN industry is today mainly boosted by newly emerging markets.

TECHCET CA, an advisory service firm providing electronic materials information, today announced that the silicon wafer supply for semiconductor device fabrication is forecasted to appreciably lag demand starting next year, and could remain in shortage through the year 2021 despite investments in China. Silicon wafer area demand is forecasted to steadily increase at a CAGR of ~3.1% over the 2016-2021 period to reach over 13,000 million square inches (MSI). Executives of silicon wafer suppliers have stated that average selling prices have remained too low to allow for investment in 300mm expansions, as detailed in a quarterly update to the TECHCET Critical Materials Report, “Silicon Wafers Market & Supply-Chain.”

The silicon wafer supply-chain is dominated by two suppliers–Shin-Etsu Handotai and SUMCO–combining to capture almost two-thirds of the global wafer market in 2016, and the top five representing over 92% of total revenues. The silicon wafer market is maturing as evidenced by recent mergers and acquisitions, the two most notable being the acquisition of SunEdison Semi by GlobalWafers (Taiwan) and the assumption of majority ownership of LG Siltron by SK Holdings (Korea).

“Over the last five years, the average selling price per square inch of semiconductor-grade silicon wafers has declined by about a third and more than a half from the 2007 level,” explained Michel Walden, lead author of the report and senior technology analyst with TECHCET. “However, current tightness in the supply-chain has led to greater stability and even price increases in some cases, all of which is likely needed for the long-term health of the wafer suppliers.”

Over the past few years, silicon suppliers decommissioned roughly 25% of the peak capacity for 200mm wafers. Of the remaining 200mm capacity, roughly 65% of the total demand is for epitaxial (epi) wafers, and a series of epi service companies have embraced this opportunity and provide a variety of layer configurations for their customers.

An international team of physicists, materials scientists and string theoreticians have observed a phenomenon on Earth that was previously thought to only occur hundreds of light years away or at the time when the universe was born. This result could lead to a more evidence-based model for the understanding the universe and for improving the energy-conversion process in electronic devices.

Using a recently discovered material called a Weyl semimetal, similar to 3D graphene, scientists at IBM Research (NYSE: IBM) have mimicked a gravitational field in their test sample by imposing a temperature gradient. The study was supervised by Prof. Kornelius Nielsch, Director at the Leibniz Institute for Materials and Solid State Research Dresden (IFW) and Prof. Claudia Felser, Director at the Max Planck Institute for Chemical Physics of Solids in Dresden.

After conducting the experiment in a cryolab at the University of Hamburg with high magnetic fields, a team of theoreticians from TU Dresden, UC Berkeley and the Instituto de Fisica Teorica UAM/CSIC confirmed with detailed calculations that they observed a quantum effect known as an axial-gravitational anomaly, which breaks one of the classical conservation laws, such as charge, energy and momentum.

This law-breaking anomaly had previously been derived in purely theoretical reasoning with methods based on string theory. It was believed to exist only at extremely high temperatures of trillions of degrees, as an exotic form of matter, called a quark-gluon plasma, at the early stages of the universe deep within the cosmos or created using particle colliders. But to their surprise, the researchers discovered that it also exists on Earth in the properties of solid-state physics, on which much of the computing industry is based on, spanning from tiny transistors to cloud data centers. This discovery is appearing today in the peer-reviewed journal Nature.

“For the first time, we have experimentally observed this fundamental quantum anomaly on Earth which is extremely important towards our understanding of the universe,” said Dr. Johannes Gooth, an IBM Research scientist and lead author of the paper. “We can now build novel solid-state devices based on this anomaly that have never been considered before to potentially circumvent some of the problems inherent in classical electronic devices, such as transistors.”

“This is an incredibly exciting discovery. We can clearly conclude that the same breaking of symmetry can be observed in any physical system, whether it occurred at the beginning of the universe or is happening today, right here on Earth,” said Prof. Dr. Karl Landsteiner, a string theorist at the Instituto de Fisica Teorica UAM/CSIC and co-author of the paper.

IBM scientists predict this discovery will open up a rush of new developments around sensors, switches and thermoelectric coolers or energy-harvesting devices, for improved power consumption.

A little fluorine turns an insulating ceramic known as white graphene into a wide-bandgap semiconductor with magnetic properties. Rice University scientists said that could make the unique material suitable for electronics in extreme environments.

A proof-of-concept paper from Rice researchers demonstrates a way to turn two-dimensional hexagonal boron nitride (h-BN) – aka white graphene – from an insulator to a semiconductor. The magnetism, they said, is an unexpected bonus.

Because the atomically thin material is an exceptional conductor of heat, the researchers suggested it may be useful for electronics in high-temperature applications, perhaps even as magnetic memory devices.

The discovery appears this week in Science Advances.

“Boron nitride is a stable insulator and commercially very useful as a protective coating, even in cosmetics, because it absorbs ultraviolet light,” said Rice materials scientist Pulickel Ajayan, whose lab led the study. “There has been a lot of effort to try to modify its electronic structure, but we didn’t think it could become both a semiconductor and a magnetic material.

“So this is something quite different; nobody has seen this kind of behavior in boron nitride before,” he said.

The researchers found that adding fluorine to h-BN introduced defects into its atomic matrix that reduced the bandgap enough to make it a semiconductor. The bandgap determines the electrical conductivity of a material.

“We saw that the gap decreases at about 5 percent fluorination,” said Rice postdoctoral researcher and co-author Chandra Sekhar Tiwary. The gap gets smaller with additional fluorination, but only to a point. “Controlling the precise fluorination is something we need to work on. We can get ranges but we don’t have perfect control yet. Because the material is atomically thin, one atom less or more changes quite a bit.

“In the next set of experiments, we want to learn to tune it precisely, atom by atom,” he said.

They determined that tension applied by invading fluorine atoms altered the “spin” of electrons in the nitrogen atoms and affected their magnetic moments, the ghostly quality that determines how an atom will respond to a magnetic field like an invisible, nanoscale compass.

“We see angle-oriented spins, which are very unconventional for 2-D materials,” said Rice graduate student and lead author Sruthi Radhakrishnan. Rather than aligning to form ferromagnets or canceling each other out, the spins are randomly angled, giving the flat material random pockets of net magnetism. These ferromagnet or anti-ferromagnet pockets can exist in the same swatch of h-BN, which makes them “frustrated magnets” with competing domains.

The researchers said their simple, scalable method can potentially be applied to other 2-D materials. “Making new materials through nanoengineering is exactly what our group is about,” Ajayan said.

Co-authors of the paper are graduate students Carlos de los Reyes and Zehua Jin, chemistry lecturer Lawrence Alemany, postdoctoral researcher Vidya Kochat and Angel Martí, an associate professor of chemistry, of bioengineering and of materials science and nanoengineering, all of Rice; Valery Khabashesku of Rice and the Baker Hughes Center for Technology Innovation, Houston; Parambath Sudeep of Rice and the University of Toronto; Deya Das, Atanu Samanta and Rice alumnus Abhishek Singh of the Indian Institute of Science, Bangalore; Liangzi Deng and Ching-Wu Chu of the University of Houston; Thomas Weldeghiorghis of Louisiana State University and Ajit Roy of the Air Force Research Laboratories at Wright-Patterson Air Force Base.

Ajayan is chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry.

TECHCET CA—the advisory service firm providing electronic materials information—today announced that specialty chemical precursor market for the deposition of dielectrics and metals in integrated circuit (IC) fabrication is forecasted to increase at ~10% CAGR through the year 2021. TECHCET’s proprietary Wafer Forecast Model (WFM) shows that 3D-NAND devices are expected to grow at a rapid pace from 2016 and become one of the top three market segments by 2020. Logic ICs will continue to evolve, from 3D finFET devices to Gate-All-Around Nano-Wires (GAA-NW), enabled by new critical materials and manufacturing processes as detailed in new reports from TECHCET, “Advanced Insulating Dielectric Precursors,” and “ALD/CVD High-k & Metal Precursors.”

Precursors tracked by TECHCET for ALD/CVD of metal and high-k dielectric films on IC wafers include sources of aluminum, cobalt, hafnium, tantalum, titanium, tungsten, and zirconium. The total market for 2017 is now estimated to be US$435M, growing to US$638M in 2021. The top-2 suppliers are estimated to hold more than half of the total available market, with many players competing to supply the next enabling molecule. In particular, cobalt precursor demand is forecasted to reach >$80M in 2021 as foundries transition to below 14nm-node processing. As a potential conflict mineral, TECHCET tracks the sub-suppliers of cobalt.

“Metal precursors have had double-digit growth over an extended period of time, and we expect that to continue as the IC industry transitions to 10nm- and 7nm-node logic and 3D-NAND fabrication, with an average long term CAGR of 11% over 2013 to 2021,” says Dr. Jonas Sundqvist, lead author of the report, senior technology analyst with TECHCET and researcher with Fraunhofer IKTS. “Dielectric precursors growth today is clearly driven by dielectric PEALD deposition in multiple patterning, and by dielectric CVD in 3D-NAND.”

Precursors tracked by TECHCET for ALD/CVD/SOD of advanced dielectric films on IC wafers include multiple sources of silicon. The total market for 2017 is now estimated to be just over US$400M, growing to US$560M in 2021. Current growth over 10% is expected to slow slightly to be in the 8-10% range over 2019-2021. Anticipated near-term developments include transitions from CVD to ALD for several IC fab modules.

By Pete Singer

In order to increase device performance, the semiconductor industry has slowly been implementing many new materials. From the 1960s through the 1990s, only a handful of materials were used, most notably silicon, silicon oxide, silicon nitride and aluminum. Soon, by 2020, more than 40 different materials will be in high-volume production, including more “exotic” materials such as hafnium, ruthenium, zirconium, strontium, complex III-Vs (such as InGaAs), cobalt and SiC.

These new materials create a variety of challenges with regard to process integration (understanding material interface issues, adhesion, stress, cross-contamination, etc.). But they also create new challenges when it comes to material handling.

“As we go through technology node advancements, people are looking at the potential of different materials on the wafer,” notes Clint Haris, Senior Vice President and General Manager of the Microcontamination Control Division at Entegris (Billerica, MA). “They’re looking at different chemicals that are required to clean those materials to reduce defects and improve their operational yield, and what we’re increasingly seeing is that fabs are concerned with the fact that contamination can be introduced in the fluid stream anywhere in that long process flow.”

Haris said that part of their mission at Entegris is to make sure that the entire supply chain – from the development of a chemistry at the supplier to its use on a wafer in a fab – is working in harmony, particularly with regard to any materials that might “touch” the chemicals. “Not only do you want to filter and purify things throughout the whole fluid flow,” he said, “but you want to have that last filtration right before the fluid touches the surface of the wafer.”

The goal of filtration is, of course, to remove contaminants and particles before they reach the wafer, but the exact purity required can be a moving target. “Today we’re seeing a lot of these materials and liquids, which have a parts per trillion purity level, but there’s a desire to move to parts per quadrillion,” Haris said. That’s the equivalent of one drop in all the water that flows over Niagra Falls in one day.

In addition to the filtration challenge of achieving that level, there’s the question of do the analytical tools exist to actually measure contaminants at that level. The answer – not yet. “It’s actually a real issue where some of the metrology tools cannot meet our customers’ needs at those levels, and so one of the things that we’ve done is we’ve developed some techniques internally to enhance the capability of metrology,” Haris said. “We also work on how we prepare our samples so you can detect contamination at those levels.” Because that level of detection is so difficult — in some cases impossible – Haris said fabs are increasingly putting additional filters at the process tool and at the dispense nozzle to “protect against the unknown.”

Earlier this year, Entegris introduced Purasol™, a first-of-its-kind solvent purifier that removes a wide variety of metal microcontaminants found in organic solvents used in ultraclean chemical manufacturing processes. Using tailored membrane technology, the purifier can efficiently remove both dissolved and colloidal metal contaminants from a wide variety of ultra-pure, polar and non-polar solvents. “One of the main things that our customers are seeing is a concern with metal contamination in the photo process that can result in particular defects (see Figure), such as bridge defects,” Haris explained. Increasingly, fabs are moving from just filtration (removing particles) to purification (removing ions and metals), he added.

Illustration of metal contamination inducing defects on lithography process.

Illustration of metal contamination inducing defects on lithography process.

Entegris also recently acquired W. L. Gore & Associates’ water and chemical filtration product line for microelectronics applications. “This is a Teflon-based product line, which is used in ultrapure water filtration for semiconductor fabs, but it’s also a product that we’re selling into some of the fine chemical purification markets for some of the chemistries that are brought into the fabs,” Haris said. “We are focused on new product development and M&A to enhance our capability to support our customers as they overcome these contamination challenges..”

A new low-temperature solution printing technique allows fabrication of high-efficiency perovskite solar cells with large crystals intended to minimize current-robbing grain boundaries. The meniscus-assisted solution printing (MASP) technique boosts power conversion efficiencies to nearly 20 percent by controlling crystal size and orientation.

The process, which uses parallel plates to create a meniscus of ink containing the metal halide perovskite precursors, could be scaled up to rapidly generate large areas of dense crystalline film on a variety of substrates, including flexible polymers. Operating parameters for the fabrication process were chosen by using a detailed kinetics study of perovskite crystals observed throughout their formation and growth cycle.

“We used a meniscus-assisted solution printing technique at low temperature to craft high quality perovskite films with much improved optoelectronic performance,” said Zhiqun Lin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “We began by developing a detailed understanding of crystal growth kinetics that allowed us to know how the preparative parameters should be tuned to optimize fabrication of the films.”

The new technique is reported July 7 in the journal Nature Communications. The research has been supported by the Air Force Office of Scientific Research (AFOSR) and the National Science Foundation (NSF).

Georgia Tech Research Scientist Ming He adjusts the equipment for the meniscus-assisted solution printing (MASP) technique used to fabricate perovskite films for solar cells. (Credit: Rob Felt, Georgia Tech)

Georgia Tech Research Scientist Ming He adjusts the equipment for the meniscus-assisted solution printing (MASP) technique used to fabricate perovskite films for solar cells. (Credit: Rob Felt, Georgia Tech)

Perovskites offer an attractive alternative to traditional materials for capturing electricity from light, but existing fabrication techniques typically produce small crystalline grains whose boundaries can trap the electrons produced when photons strike the materials. Existing production techniques for preparing large-grained perovskite films typically require higher temperatures, which is not favorable for polymer materials used as substrates – which could help lower the fabrication costs and enable flexible perovskite solar cells.

So Lin, Research Scientist Ming He and colleagues decided to try a new approach that relies on capillary action to draw perovskite ink into a meniscus formed between two nearly parallel plates approximately 300 microns apart. The bottom plate moves continuously, allowing solvent to evaporate at the meniscus edge to form crystalline perovskite. As the crystals form, fresh ink is drawn into the meniscus using the same physical process that forms a coffee ring on an absorbent surface such as paper.

“Because solvent evaporation triggers the transport of precursors from the inside to the outside, perovskite precursors accumulate at the edge of the meniscus and form a saturated phase,” Lin explained. “This saturated phase leads to the nucleation and growth of crystals. Over a large area, we see a flat and uniform film having high crystallinity and dense growth of large crystals.”

To establish the optimal rate for moving the plates, the distance between plates and the temperature applied to the lower plate, the researchers studied the growth of perovskite crystals during MASP. Using movies taken through an optical microscope to monitor the grains, they discovered that the crystals first grow at a quadratic rate, but slow to a linear rate when they began to impinge on their neighbors.

“When the crystals run into their neighbors, that affects their growth,” noted He. “We found that all of the grains we studied followed similar growth dynamics and grew into a continuous film on the substrate.”

The MASP process generates relatively large crystals – 20 to 80 microns in diameter – that cover the substrate surface. Having a dense structure with fewer crystals minimizes the gaps that can interrupt the current flow, and reduces the number of boundaries that can trap electrons and holes and allow them to recombine.

Using films produced with the MASP process, the researchers have built solar cells that have power conversion efficiencies averaging 18 percent – with some as high as 20 percent. The cells have been tested with more than 100 hours of operation without encapsulation. “The stability of our MASP film is improved because of the high quality of the crystals,” Lin said.

Doctor-blading is one of the conventional perovskite fabrication techniques in which higher temperatures are used to evaporate the solvent. Lin and his colleagues heated their substrate to only about 60 degrees Celsius, which would be potentially compatible with polymer substrate materials.

So far, the researchers have produced centimeter-scale samples, but they believe the process could be scaled up and applied to flexible substrates, potentially facilitating roll-to-roll continuous processing of the perovskite materials. That could help lower the cost of producing solar cells and other optoelectronic devices.

“The meniscus-assisted solution printing technique would have advantages for flexible solar cells and other applications requiring a low-temperature continuous fabrication process,” Lin added. “We expect the process could be scaled up to produce high throughput, large-scale perovskite films.”

Among the next steps are fabricating the films on polymer substrates, and evaluating other unique properties (e.g., thermal and piezotronic) of the material.

A new type of semiconductor may be coming to a high-definition display near you. Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have shown that a class of semiconductor called halide perovskites is capable of emitting multiple, bright colors from a single nanowire at resolutions as small as 500 nanometers.

A 2-D plate showing alternating cesium lead chloride (blue) and cesium lead bromide (green) segments. Credit: Letian Dou/Berkeley Lab and Connor G. Bischak/UC Berkeley

A 2-D plate showing alternating cesium lead chloride (blue) and cesium lead bromide (green) segments. Credit: Letian Dou/Berkeley Lab and Connor G. Bischak/UC Berkeley

The findings, published online this week in the early edition of the Proceedings of the National Academy of Sciences, represent a clear challenge to quantum dot displays that rely upon traditional semiconductor nanocrystals to emit light. It could also influence the development of new applications in optoelectronics, photovoltaics, nanoscopic lasers, and ultrasensitive photodetectors, among others.

The researchers used electron beam lithography to fabricate halide perovskite nanowire heterojunctions, the junction of two different semiconductors. In device applications, heterojunctions determine the energy level and bandgap characteristics, and are therefore considered a key building block of modern electronics and photovoltaics.

The researchers pointed out that the lattice in halide perovskites is held together by ionic instead of covalent bonds. In ionic bonds, atoms of opposite charges are attracted to each other and transfer electrons to each other. Covalent bonds, in contrast, occur when atoms share their electrons with each other.

“With inorganic halide perovskite, we can easily swap the anions in the ionic bonds while maintaining the single crystalline nature of the materials,” said study principal investigator Peidong Yang, senior faculty scientist at Berkeley Lab’s Materials Sciences Division. “This allows us to easily reconfigure the structure and composition of the material. That’s why halide perovskites are considered soft lattice semiconductors. Covalent bonds, in contrast, are relatively robust and require more energy to change. Our study basically showed that we can pretty much change the composition of any segment of this soft semiconductor.”

In this case, the researchers tested cesium lead halide perovskite, and then they used a common nanofabrication technique combined with anion exchange chemistry to swap out the halide ions to create cesium lead iodide, bromide, and chloride perovskites.

Each variation resulted in a different color emitted. Moreover, the researchers showed that multiple heterojunctions could be engineered on a single nanowire. They were able to achieve a pixel size down to 500 nanometers, and they determined that the color of the material was tunable throughout the entire range of visible light.

The researchers said that the chemical solution-processing technique used to treat this class of soft, ionic-bonded semiconductors is far simpler than methods used to manufacture traditional colloidal semiconductors.

“For conventional semiconductors, fabricating the junction is quite complicated and expensive,” said study co-lead author Letian Dou, who conducted the work as a postdoctoral fellow in Yang’s lab. “High temperatures and vacuum conditions are usually involved to control the materials’ growth and doping. Precisely controlling the materials composition and property is also challenging because conventional semiconductors are ‘hard’ due to strong covalent bonding.”

To swap the anions in a soft semiconductor, the material is soaked in a special chemical solution at room temperature.

“It’s a simple process, and it is very easy to scale up,” said Yang, who is also a professor of chemistry at UC Berkeley. “You don’t need to spend long hours in a clean room, and you don’t need high temperatures.”

The researchers are continuing to improve the resolution of these soft semiconductors, and are working to integrate them into an electric circuit.

A multi-institutional team led by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) discovered a way to create new alloys that could form the basis of next-generation semiconductors.

Semiconductor alloys already exist-often made from a combination of materials with similar atomic arrangements-but until now researchers believed it was unrealistic to make alloys of certain constituents.

“Maybe in the past scientists looked at two materials and said I can’t mix those two. What we’re saying is think again,” said Aaron Holder, a former NREL post-doctoral researcher and now research faculty at the University of Colorado Boulder. Holder is corresponding author of a new paper in Science Advances titled Novel phase diagram behavior and materials design in heterostructural semiconductor alloys. “There is a way to do it.”

Scientists connected to the Center for Next Generation of Materials by Design (CNGMD) made the breakthrough and took the idea from theory to reality. An Energy Frontier Research Center, which is supported by the Energy Department’s Office of Science and researchers from NREL, the Colorado School of Mines, Harvard University, Lawrence Berkeley National Laboratory, Massachusetts Institute of Technology, Oregon State University, and SLAC National Accelerator Laboratory.

“It’s a really nice example of what happens when you bring different institutions with different capabilities together,” said Holder. His co-authors from NREL are Stephan Lany, Sebastian Siol, Paul Ndione, Haowei Peng, William Tumas, John Perkins, David Ginley, and Andriy Zakutayev.

A mismatch between atomic arrangements previously thwarted the creation of certain alloys. Researchers with CNGMD were able to create an alloy of manganese oxide (MnO) and zinc oxide (ZnO), even though their atomic structures are very different. The new alloy will absorb a significant fraction of natural sunlight, although separately neither MnO nor ZnO can. “It’s a very rewarding kind of research when you work as a team, predict a material computationally, and make it happen in the lab,” Lany said.

Using heat, blending a small percent of MnO with ZnO already is possible, but reaching a 1:1 mix would require temperatures far greater than 1,000 degrees Celsius (1,832 degrees Fahrenheit), and the materials would separate again as they cool.

The scientists, who also created an alloy of tin sulfide and calcium sulfide, deposited these alloys as thin films using pulsed laser deposition and magnetron sputtering. Neither method required such high temperatures. “We show that commercial thin film deposition methods can be used to fabricate heterostructural alloys, opening a path to their use in real-world semiconductor applications,” co-author Zakutayev said.

The research yielded a first look at the phase diagram for heterostructural alloys, revealing a predictive route for properties of other alloys along with a large area of metastability that keeps the elements combined. “The alloy persists across this entire space even though thermodynamically it should phase separate and decompose,” Holder said.